Method for treating slags or slag mixtures on an iron bath

ABSTRACT

In a method for treating slags or slag mixtures having iron oxide contents of &gt;5 wt.-%, in particular steelworks slags, in which the steel slags optionally mixed with other slags are charged onto a metal bath, a steel bath having a carbon content of &lt;1.5 wt.-%, preferably &lt;0.5 wt.-%, is used as the metal bath and the steel bath, after the charging of the steel slags, is carburized to above 2.0 wt.-% C, preferably &gt;2.5 wt.-% C, by introducing carbon or carbon carriers.

The invention relates to a method for-treating slags or slag mixtureshaving iron oxide contents of >5 wt.-%, in particular steelworks slags,in which the steel slags optionally mixed with other slags are chargedonto a metal bath.

From EP 666 930 B1, a method for producing steel and hydraulicallyactive binders has already been known, by which steel slags are reducedusing pig iron and, in particular, the carbon content present in the pigiron, thus causing the refining of the pig iron bath and the reductionof the carbon content of the bath to, for instance, below half of thecarbon content present in the pig iron, on the one hand while, at thesame time, causing the iron oxide from the steel slag to be reduced toiron and get into the metal bath. The known method was optimizedsubstantially with a view to enabling the at least partial reduction oflarge quantities of steel slags using slight amounts of molten pig iron.A substantial curtailment of the method by using larger quantities ofmolten pig iron is not readily feasible with that known method, wherein,for one part, molten pig iron is usually present at comparatively lowtemperatures thus eventually rendering problematic the Theologicalproperties of slags and, for the other part, when charging liquid steelslags onto large quantities of molten pig iron, extremely vigorousreactions will be observed, which, in the event of unfavourableTheological properties of the slags, may provoke the formation ofundesired foamed slags or the occurrence of slag spittings. Theformation of such foamed slags, as a result, will cause a retardation ofthe reaction such that relatively long treatment times will becomenecessary.

The invention aims to further develop a method of the initially definedkind to the extent that with a relatively short reaction time the methodheat formed in the course of the method can be optimally utilized and ahighly fluid steel slag can be maintained during the reduction, whichwill persistently impede the formation of foamed slags and, at the sametime, prevent local overreactions involving undesired liquid slagspittings.

To solve this object, the method according to the invention essentiallyconsists in that a steel bath having a carbon content of <1.5 wt.-%,preferably <0.5 wt.-%, is used as said metal bath and that the steelbath, after the charging of the steel slags, is carburized to above 2.0wt.-% C, preferably >2.5 wt.-% C, by introducing carbon or carboncarriers. By providing a metal bath having a comparatively low carboncontent, i.e. a steel bath, at the onset of the slag reduction, localoverreactions and vigorous reactions causing large amounts of gas to beformed will be prevented such that the formation of foamed slag will nolonger be observed. In order to conduct such a method in an economicaland largely autothermical manner, it is particularly advantageous if themethod heat forming in the method is used immediately. Therefore, themethod according to the invention advantageously is carried out in amanner that a pig iron bath is provided and refined with oxygen to acarbon content of <0.5 wt.-%, whereby a bath temperature of above 1570°C., in particular about 1620° C., is adjusted, that liquid steel slag ischarged onto the refined steel bath and carbon is introduced into thebath after a temperature equalization, wherein SiO₂-containingcorrective substances such as, e.g., blast furnace slag, quartz sandand/or Al₂O₃-containing corrective substances such as, e.g., bauxite areadded in order to lower the basicity to <1.5 and to adjust an Al₂O₃content of >10 wt.-%, respectively. By ensuring the formation of a steelbath onto which the steel slags are subsequently charged, immediately asa prestage within the same method by means of a refining procedure, themethod heat forming during refining, by which the original pig iron bathis substantially heated, can be directly utilized for a temperatureequalization with the steel slag to be applied, wherein it is feasible,due to the high temperature, to immediately melt down, and introduceinto the slag, also corrective substances and, in particular,SiO₂-containing corrective substances as are required for the adjustmentof the desired basicity. Onto the steel bath which is appropriatelyheated to temperatures of above 1570° C. by the refining procedure is,thus, immediately added either at least a portion of the SiO₂-containingcorrective substances required for basicity adjustment so as to causethese corrective substances to be heated and at least partially melted,or the steel slag, wherein it is feasible to add the SiO₂-containingcorrective substances together with the liquid steel slag. Due to theaddition of such SiO₂-containing corrective substances and, inparticular, the opportunity to simultaneously charge additives such as,for instance, cooling scrap or fine ore, the high latent heat of therefined steel bath is directly used in the process and it is feasible,by the addition of iron oxide carriers, to effect an efficienttemperature control by which iron oxides are largely reduced at the sametime and a high amount of molten iron is, thus, formed of such ironoxide carriers which are usually difficult to process, such as, forinstance, fine ores.

Following steel slag charging, the carbon content of the steel bath isthen continuously raised and the desired reduction work is performedwhile blowing carbon carriers into the steel bath. The carbon monoxideformed may subsequently be afterburned such that the method, in themain, can be operated in a largely autothermical manner and theintroduction of additional energy can be obviated. Advantageously, themethod according to the invention in that case is carried out in amanner that the basicity is adjusted to 1.1. to 1.4 and the carboncontent of the bath is adjusted to >2.5 wt.-%.

The method according to the invention advantageously is carried out in amanner that the steel slag is added to the steel bath at a weightquantity ratio of 1:3 to 1:6, preferably about 1:4, whereby the meltingheat required for fluxes will be immediately available due to therelatively large quantity of metal bath charged and exhibiting a hightemperature level after the refining procedure. In particular, it isfeasible in an advantageous manner to add quartz sand in amounts rangingfrom 150 to 250 kg/ton steel slag, and bauxite in amounts ranging from200 to 300 kg/ton steel slag, whereby the selected way of additioncauses the fluxes to be sufficiently homogenized and hence completelymelted and reacted in the slag so as to directly yield acement-technologically usable product.

In a particularly advantageous manner, the liquid slag mixtures aresupplemented with fine ores or iron oxide carriers in order to adjust aniron oxide content of above 8 wt.-%, whereby it is feasible,simultaneously with the desired reduction of the steel slag, to reduceeven hardly processible ores by the same method and use this admixturefor controlling the desired reaction temperatures. For the same purpose,additives such as, e.g., cooling scrap or fine ores may advantageouslybe melted down in the metal bath during or after refining.

In a particularly advantageous manner, acidic-gangue iron oxide carriersmay be added, thus enabling the appropriate lowering of the basicity tocement-technologically desirable target basicities simultaneously withthe reduction of metallic iron from such iron oxide carriers. In doingso, it is advantageously proceeded such that the addition of iron oxidecarriers such as, e.g., acidic gangue weak ores or fine ores, after theaddition of the molten slags or slag mixtures onto the steel bath, atleast partially is effected at the same time as the carburization of thesteel bath, corrective substances containing CaO, Al₂O₃ and/or SiO₂, ingeneral, being advantageously added to the molten slags or slagmixtures.

In the main, the refining procedure carried out in the first stageyields a particularly favourable energy balance with the high energyamounts released during the refining of the pig iron bath being directlyusable within the process.

Any desired slags rich in SiO₂ are basically suitable for the adjustmentof the desired target basicity and SiO₂-containing corrective substancesmay optionally be charged.

In the following, the invention will be explained in more detail by wayof an exemplary embodiment.

In a converter, 8 tons of molten pig iron were converted into a steelbath by the introduction of 280 Nm³ oxygen through bottom tuyeres. Thedirectional analysis of the molten pig iron showed a carbon content of3.9 wt.-%, a silicon content of about 0.3 wt.-% and residual iron. Afterrefining, during which, departing from a bath temperature of 147° C., afinal temperature of 1620° C. was reached due to the exothermic reactionoccurring, the liquid steel bath had a carbon content of 0.3 wt.-%, asilicon content of 0.003-% and residual iron. After this, 2 tons ofliquid steel slag were charged onto that liquid steel bath. The steelslag was characterized by the following analysis:

Steel slag wt. % CaO 48.6 SiO₂ 19.1 Al₂O₃ 2.2 MgO 3.2 TiO₂ 1.5 FeO 22.7MnO 2.7 CaO/SiO₂ 2.5

Due to the relatively low bath carbon content of the steel bath present,the reduction reaction of the metal oxides contained in the steel slaginvolves substantially fewer mass conversions immediately upon theaddition of the liquid steel slag. If pig iron were directly charged inthe respective quantitative ratios, high amounts of CO would be releasedquickly, which might lead to intensive slag foaming or slag spitting.

After the charging of the steel slag onto the steel bath, temperatureequalization between slag and metal bath is effected so as to enablepossibly solid slag portions to be completely reconverted into theliquid state. Temperature equalization results in temperatures of about1500° C.

After temperature equalization had been completed, 580 kg coal wereblown into the steel bath at a blowing rate of 25 kg/min; additionallyintroduced were 370 kg quartz sand at a rate of 24 kg/min and 535 kgbauxite at a rate of 28 kg/min.

A metal bath containing 4.5 wt.-% dissolved carbon at a temperature ofabout 1490° C. remained at the end of the desired reduction reaction.Depending on the corrective substances selected and the composition ofthe steel slag, the pig iron thus formed back naturally also may containrespective other metals than iron.

By means of the corrective substances, the desired target basicity asappears desirable for the cement-technological further use of thetreated slag was adjusted, on the one hand, while, at the same timeeffecting a reduction of the metal oxides to the following directionalanalysis of the treated steel slag.

Treated steel slag wt. % CaO 44.7 SiO₂ 34.6 Al₂O₃ 14.8 MgO 2.9 TiO₂ 1.8FeO 0.9 MnO 0.3 CaO/SiO₂ 1.3

The thus recovered treated steel slag could be granulated in water andused as a grinding additive in composite cements.

The heat formed during decarburization, i.e. burning of carbon, enablesthe heating and melting of the corrective substances required for theadjustment of the desired target basicity and the adjustment of thedesired composition aimed at a cement-technologically usable grindingadditive. An undesiredly high temperature level optionally developingmay again be reduced to the desired reduction temperature by theaddition of cooling scrap, fine ores or charging substances derived fromthe direct reduction of iron (DRI, HBI), thus allowing additional ironto be introduced into the metal bath.

To the extent it appears necessary, the carbon monoxide formed in thereduction may be afterburned above the slag bath, thus enabling thesimultaneous processing of an elevated amount of acidic fine oreswithout any additional introduction of energy.

1. A method for treating slags having iron oxide contents of greaterthan 5 percent by weight to form iron by reduction, comprising the stepsof: charging the slags having iron oxide contents of greater than 5percent by weight onto a steel bath having a carbon content of less than1.5 percent by weight, and adding at least one of the group consistingof carbon and carbon carriers into said steel bath, whereby said steelbath is carburized to above 2.0 percent carbon by weight.
 2. A methodaccording to claim 1, wherein said slags are charged onto a steel bathhaving a carbon content of less than 0.5 percent carbon by weight.
 3. Amethod according to claim 1, wherein said steel bath is carburized toabove 2.5 percent carbon by weight.
 4. A method according to claim 2,wherein said steel bath is carburized to above 2.5 percent carbon byweight.
 5. A method according to claim 1, wherein a pig iron bath isprovided and refined with oxygen to a carbon content of less than 0.5percent carbon by weight, whereby a bath temperature of above 1570° C.is provided, liquid steel slag is charged onto the refined steel bath,and carbon is introduced into the bath after a temperature equalization,wherein at least one SiO₂-containing corrective substance and/or atleast one Al₂O₃-containing corrective substance are added in order tolower the basicity to a value less than 1.5, and to adjust an Al₂O₃content of less than ten percent by weight, respectively.
 6. A methodaccording to claim 5, wherein said at least one SiO₂-containingcorrective substance is at least one of the group consisting of blastfurnace slag and quartz sand.
 7. A method according to claim 5, whereinsaid Al₂O₃-containing corrective substance is bauxite.
 8. A methodaccording to claim 5, wherein said bath temperature is adjusted to about1620° C.
 9. A method according to claim 1, wherein the basicity, definedas the ratio CaO/SiO₂, is adjusted to 1.1 to 1.4 and the carbon contentof the steel bath is adjusted to greater than 2.5 percent carbon byweight.
 10. A method according to claim 2, wherein the basicity, definedas the ratio CaO/SiO₂, is adjusted to 1.1 to 1.4 and the carbon contentof the bath is adjusted to greater than 2.5 percent carbon by weight.11. A method according to claim 1, wherein the slags are added to thebath at a weight quantity ratio, defined as weight of slags/weight ofthe steel bath, of 1:3 to 1:6.
 12. A method according to claim 1,wherein the slags are added to the bath at a weight quantity ratio,defined as weight of slags/weight of the steel bath, of about 1:4.
 13. Amethod according to claim 5, wherein quartz sand in amounts ranging from150 to 250 kg/ton steel slag, and bauxite in amounts ranging from 200 to300 kg/ton steel slag, are added.
 14. A method according to claim 1,wherein fine ores or iron oxide carriers are added to the molten slagmixtures in order to adjust the iron oxide content to greater than 8percent by weight.
 15. A method according to claim 5, wherein fine oresor iron oxide carriers are added to the molten slag mixtures in order toadjust the iron oxide content to greater than 8 percent by weight.
 16. Amethod according to claim 9, wherein fine ores or iron oxide carriersare added to the molten slag mixtures in order to adjust the iron oxidecontent to greater than 8 percent by weight.
 17. A method according toclaim 10, wherein fine ores or iron oxide carriers are added to themolten slag mixtures in order to adjust the iron oxide content togreater than 8 percent by weight.
 18. A method according to claim 13,wherein fine ores or iron oxide carriers are added to the molten slagmixtures in order to adjust the iron oxide content to greater than 8percent by weight.
 19. A method according to claim 1, wherein theaddition of iron oxide carriers after the addition of the molten slagsor slag mixtures onto the steel bath, at least partially is effected atthe same time as the carburization of the steel bath.
 20. A methodaccording to claim 5, wherein the addition of iron oxide carriers afterthe addition of the molten slags or slag mixtures onto the steel bath,at least partially is effected at the same rime as the carburization ofthe steel bath.
 21. A method according to claim 9, wherein the additionof iron oxide carriers after the addition of the molten slags or slagmixtures onto the steel bath, at least partially is effected at the sametime as the carburization of the steel bath.
 22. A method according toclaim 1, wherein corrective substances containing CaO, Al₂O₃, and/orSiO₂ are added to the molten slags or slag mixtures.
 23. A methodaccording to claim 5, wherein corrective substance containing CaO,Al₂O₃, and/or SiO₂ are added to the molten slags or slag mixtures.
 24. Amethod according to claim 9, wherein corrective substances containingCaO, Al₂O₃, and/or SiO₂ are added to the molten slags or slag mixtures.25. A method according to claim 1, wherein one or more additivesselected from the group consisting of cooling scrap, fine ores andsponge iron are melted down in the metal bath during or after refining.26. A method according to claim 5, wherein one or more additivesselected from the group consisting of cooling scrap, fine ores andsponge iron are melted down in the metal bath during or after refining.27. A method according to claim 9, wherein one or more additivesselected from the group consisting of cooling scrap, fine ores andsponge iron are melted down in the metal bath during or after refining.28. A method according to claim 13, wherein one or more additivesselected from the group consisting of cooling scrap, fine ores andsponge iron are melted down in the metal bath during or after refining.29. A method according to claim 1, wherein the stags having iron oxidecontents of greater than 5 percent by weight are steelworks slags.
 30. Amethod according to claim 19, wherein the iron oxide carriers are one ormore selected from the group consisting of acidic gangue weak ores andfine ores.
 31. A method according to claim 20, wherein the iron oxidecarriers are one or more selected from the group consisting of acidicgangue weak ores and fine ores.
 32. A method according to claim 21,wherein the iron oxide carriers are one or more selected from the groupconsisting of acidic gangue weak ores and fine ores.